Micropuncture directly into the loop of Henle indicated that in Magnesium-loaded rats, more Mg was found at the site than the load filtered by the glomerulus, indicating that some Mg was entering the tubular lumen prior to the bend of the loop of Henle. Since the proximal tubule is found to reabsorb rather than secrete Mg, the site of Mg backflux is either the pars recta or the descending limb of the loop of Henle. The purpose of our proposal is to further investigate the site of this Mg influx and the contribution of the collecting duct to an eventual countercurrent mechanism for Mg, in the rat papilla. The proposed work consists of measuring Mg concentration in the accessible part of the loop of Henle and in the adjacent vasa recta, in order to determine and quantify the concentration gradient between these two structures. We are also measuring tubular fluid Mg Content at two sites of the same collecting duct to detect Mg transport at this level. No Mg transport is found in the collecting duct. Moreover, Mg concentration is higher in the descending limb of the loop of Henle than in the adjacent vasa recta, in both control and Mg-loaded animals. These findings suggest but do not prove that in Mg-loaded animals, Mg influx might occur in the pars recta. We therefore intend to study bidirectional Mg flux in the pars recta.